[The calcium hypothesis of brain aging]

The "calcium hypothesis of brain aging" assumes that a small increase in free intra-cellular calcium concentration ([Ca2+]i) over years or decades finally leads to brain lesions similar to the short [Ca2+]i overload following one acute event (e.g., stroke). Recent data are reviewed that disprove the hypothesis in this rather simple form. Studies on brain cells of experimental animals as well as on animal and human blood cells suggest that [Ca2+]i is reduced rather than elevated in brain aging. However, probably as compensation, aging seems to lead to enhanced sensitivity of the brain (or of calcium-dependent mechanisms in the brain) to changes in [Ca2+]i. Under normal conditions, both alterations seem to compensate each other. However, under situations of additional stress leading to elevated [Ca2+]i (hypoxia, hypoglycemia), aged brain cells might be more vulnerable because of a reduced ability to down-regulate [Ca2+]i. In contrast to these typical changes in the aging, very little evidence exists that [Ca2+]i is also changed in Alzheimer's disease. On the other hand, recent evidence suggests that the modulation of [Ca2+]i by beta-amyloid is specifically altered in this disease, but the pathogenetic significance of this observation is not yet finally understood.